Colloidal rods in confinement

We study colloidal rods in conï¬nement and under ï¬ow on the single particle level using optical microscopy and laser scanning confocal microscopy. First, we investigate liquid crystals in conï¬nement. A mathematical method for calculating distortion energies is developed and illustrated by applying it to both artiï¬cial data and experimental results. Next, we study an isotropic phase in strong conï¬nement, and observe a capillary nematisation transition. This is the ï¬rst observation of capillary nematisation in a colloidal liquid crystal. The results are compared to the Zwanzig model and a DFT model for semi-ï¬exible rods, and we ï¬nd good agreement with both. The DFT model, in which rod orientations are unrestricted and rod flexibility is taken into account, was found to have a better quantitative agreement with the experiments. In the second half of this thesis, we study single colloidal rods ï¬owing in a plane Poiseuille ï¬ow. For the most simple case of non-magnetic Brownian rods in water, we observe aperiodic kayaking and xy-tumbling behaviour. The results are compared to Jeï¬ery theory for non-Brownian rods in a simple shear ï¬ow and to multi-particle collision dynamics simulations. The experimental results were found to undergo the same type of behaviour as predicted by Jeï¬eryâs theory, but without periodicity, and particles were seen to switch from one type of behaviour to another. We ï¬nd qualitative agreement between the experiment and simulations, which indicates that the presence of Brownian motion can explain the diï¬erences between theory and experiment. Following on, we investigate further deviations from Jeï¬ery theory by studying the eï¬ect of an external magnetic ï¬eld and of a non-Newtonian ï¬uid on the orientational behaviour of rods. We ï¬nd that a magnetic ï¬eld shows somewhat similar behaviour as the kayaking and tumbling found in the non-magnetic system, but suppressed towards a plane perpendicular to the magnetic ï¬eld. Rods ï¬owing through a non-Newtonian ï¬uid are found to be aligned in the direction of ï¬ow, and do not undergo any tumbling behaviour. The absence of tumbling can be explained by the depletion interaction between rods and the wall. It is unclear why the preferred orientation for rods is along the direction of ï¬ow.